The family of G-protein coupled receptors (GPCRs) is one of the most important classes of proteins from both functional and structural standpoints. The human genome contains nearly 950 genes coding for GPCRs, of which nearly 450 genes have been implicated as therapeutic targets. Ligand binding to GPCRs induces multiple receptor conformations and different ligands may stabilize distinct receptor conformations (Kenakin & Miller, 2010, Pharmacol. Rev. 62(2):265-304). The concept of functional selectivity is based on the hypothesis that distinct receptor conformations recruit distinct signaling proteins, leading to preferential activation of one signaling pathway over another (Mailman, 2007, Trends Pharmacol. Sci. 28(8):390-396). In addition to selecting the signaling pathways, agonist-induced receptor conformations can also potentially affect receptor signaling properties.
Among the GPCRs, the subfamily of dopamine receptors has attracted attention from biologists and pharmacologists. In the central nervous system, dopamine receptors are widely expressed and involved in the control of locomotion, cognition, emotion and neuroendocrine secretion. In the peripheral system, dopamine receptors are present more prominently in kidney, vasculature and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. While there are numerous examples of functionally-selective ligands that activates one signaling cascade preferentially over others, functionally-selective ligands that alter receptor signaling properties are rare and have not been described for dopamine receptors.
The neurotransmitter dopamine controls a wide variety of physiological and behavioral functions in mammals via five major subtypes of dopamine receptors. They are broadly classified into the “D1-like” and “D2-like” dopamine receptors based on pharmacology and function. The D1-like receptors consist of D1 and D5 receptors, while the D2-like receptors consist of D2, D3 and D4 receptors.
The D3 receptor primarily couples to the pertussis toxin-sensitive Gα-proteins (Gi/Go) (Ahlgren-Beckendorf & Levant, 2004, J. Recept. Signal Transduct. Res. 24(3):117-130). When transfected into different cell lines, the D3 receptor couples to adenylyl cyclase V isoform (Robinson & Caron, 1997, Mol. Pharmacol. 52:508-514) and initiates signaling events including phosphorylation of mitogen-activated protein (MAP) kinases (Cussac et al., 1999, Mol. Pharmacol. 56(5):1025-103). D2 and D3 dopamine receptors also modulate potassium and calcium channel function (Seabrook et al., 1994, Br. J. Pharmacol. 111:391-393; Werner et al., 1996, Mol. Pharmacol. 49:656-661). Transfected D3 receptors couple robustly to natively expressed G-protein coupled inward rectifier potassium (GIRK) and voltage-gated P/Q type calcium channels, and inhibit firing of spontaneous action potentials and secretory activity in the AtT-20 neuroendocrine cell line (Kuzhikandathil & Oxford, 1999, J. Neurosci. 19(5):1698-1707; Kuzhikandathil & Oxford, 2000, J. Gen. Physiol. 115:697-706; Kuzhikandathil et al., 1998, Mol. Cell Neurosci. 12:390-402). The D3 receptor further couples to natively expressed adenylyl cyclase V (Kuzhikandathil & Bartoszyk, 2006, Neuropharm. 51:873-884), MAP kinases (Westrich & Kuzhikandathil, 2007, Biochim. Biophys. Acta-MCR 1773:1747-1758) and ion channels (Kuzhikandathil & Oxford, 1999, J. Neurosci. 19(5):1698-1707; Kuzhikandathil & Oxford, 2000, J. Gen. Physiol. 115:697-706; Kuzhikandathil et al., 1998, Mol. Cell Neurosci. 12:390-402; Kuzhikandathil et al., 2004, Mol. Cell Neurosci. 26:144-155) in AtT-20 cells.
Dopamine receptors are targets for the treatment of various neurological and psychiatric disorders, such as Parkinson's Disease, schizophrenia, drug addiction, depression, bipolar disorder, attention deficit hyperactivity syndrome, Tourette's Syndrome, Huntington's Disease and migraine.
Norepinephrine, also known as noradrenaline or 4-[(1R)-2-amino-1-hydroxy ethyl]benzene-1,2-diol, is a catecholamine that acts as a hormone and a neurotransmitter. Norepinephrine is the hormone and neurotransmitter most responsible for concentration, in contrast to the chemically similar hormone dopamine, also known as 4-(2-aminoethyl) benzene-1,2-diol), which is most responsible for alertness. Areas of the body that produce or are affected by norepinephrine are described as noradrenergic.
Norepinephrine has an important role as the neurotransmitter released from the sympathetic neurons to increase the rate of contractions in the heart. As a stress hormone, norepinephrine affects parts of the brain, such as the amygdala, where attention and responses are controlled. Along with epinephrine (also known as adrenaline or (R)-4-(1-Hydroxy-2-(methylamino)ethyl)benzene-1,2-diol), norepinephrine underlies the fight-or-flight response, directly increasing heart rate, triggering the release of glucose from energy stores, and increasing blood flow to skeletal muscle. It increases the brain's oxygen supply, and may also suppress neuro-inflammation when released diffusely in the brain from the locus coeruleus. As a drug, norepinephrine increases blood pressure by increasing vascular tone (tension of vascular smooth muscle) through α-adrenergic receptor activation.
Norepinephrine has potentially beneficial effects on attention deficit/hyperactivity disorder, depression and hypotension, but, as with other catecholamines, it cannot be used in the clinic because it does not cross the blood-brain barrier. However, drugs that inhibit norepinephrine transporter in the prefrontal cortex (PFC), such as methylphenidate (MPH, also known as methyl phenyl(piperidin-2-yl)acetate), increase extracellular concentrations of norepinephrine in brain tissue and increase rodent performance in a sustained attention task.
Non-norepinephrine drugs such as amphetamines are used to stimulate brain activity levels. For people with attention-deficit/hyperactivity disorder (ADHD), psychostimulant medications such as amphetamines (ADDERALL® or DESOXYN®) are prescribed to increase both levels of norepinephrine and dopamine. Methylphenidate (RITALIN® or CONCERTA®, a dopamine reuptake inhibitor) and atomoxetine (STRATTERA® or (3R)—N-methyl-3-(2-methylphenoxy)-3-phenylpropan-1-amine; a selective norepinephrine reuptake inhibitor) increase both norepinephrine and dopamine in the prefrontal cortex equally, but only dopamine (in the case of methylphenidate) and norepinephrine (in the case of atomoxetine) elsewhere in other parts of the brain. Other serotonin-norepinephrine reuptake inhibitors (SNRIs) currently approved as antidepressants are also used off-label for treatment of ADHD. The few medications available to treat ADHD, such as MPH, have been illegally used by students and teenagers as a stimulant to boost their grades.
In addition to its neurotransmitter role, the norepinephrine from locus coeruleus cells locally diffuses from “varicosities,” providing an endogenous anti-inflammatory agent in the microenvironment around the neurons, glial cells, and blood vessels in the neocortex and hippocampus. Up to 70% of norepinephrine projecting cells are lost in Alzheimer's Disease. Norepinephrine stimulates mouse microglia to suppress Aβ-induced production of cytokines and their phagocytosis of Aβ, suggesting this loss might have a role in causing this disease. There are currently no medications that may be used to treat dementia associated with a neurodegenerative disorder such as Parkinson's Disease and Alzheimer's Disease.
There is a need in the art for novel methods of treating or preventing an attention disorder and/or cognitive disorder in a subject in need thereof. Further, there is a need in the art for novel methods of treating or preventing dementia associated with a neurodegenerative disorder in a subject in need thereof. The present invention fulfills these needs.